Antenna parameters part 1: Frequency bands, Gain and Radiation Pattern

Poynting Antennas (Pty) Ltd

21 March 2016 AntennaParameters Part 1 - Frequency bands, gain and radiation pattern Page 1

ON THE PROPERTIES OF AN ANTENNA This is part of an internal document that that gives an overview of the properties of antennas for non-engineers.

We have divided the document into different posts where we discus each of the parameters:

- Frequency bands, gain and radiation pattern - Polarisation - Input Impedance and VSWR - Port to port Isolation and Cross-polarisation - Power Handling ability - Antenna “Specmanship”

Where applicable we have added some videos explaining the properties discussed. You will find a link to a PDF below.

Introduction An antenna is a device that converts energy from one form to another. When used in transmit mode, currents in the coaxial cable (feeding the antenna) flow into the antenna and the energy is converted to electromagnetic radiation which propagates into space. When an antenna is used in receive mode, electromagnetic radiation interacts with the antenna inducing currents into its components. These currents flow along the coaxial cable connected to the antenna to a receiver.

In some ways the antenna is analogous to a speaker in a sound system. A speaker converts electrical energy (from the wires powering the speaker) into sound energy which we can detect using our ears. When operated in the opposite mode a microphone is created. This device detects sound wave and converts them to electrical energy. An antenna works with electromagnetic radiation and electric currents rather than sound and electric currents.

An antenna is described by a number of attributes including frequency bands of operation, gain, radiation pattern, polarisation, VSWR, input impedance, coupling, power handling ability and so on.

This document describes each of these parameters.

1. Frequency bands of operation An antenna is designed to work over a specific frequency band or over sub-bands within a larger frequency range. Within the specified frequency bands antenna attributes such as gain, pattern and VSWR should be fairly well controlled.

2. Gain and radiation pattern The gain of an antenna represents the antenna’s maximum ability to focus electromagnetic radiation in one particular direction (when transmitting) or its ability to receive energy in that particular direction.

The radiation pattern of an antenna describes graphically how an antenna radiates or receives EM radiation over a region in space. Both the gain and radiation pattern should be considered when assessing the radiation characteristics of an antenna.



Figure 1: A 3D radiation pattern (viewed from the front and back)

Figure 1 gives an example of a radiation pattern that shows how an antenna radiates its energy over space (or receives energy from various directions). The scale on the RHS of the plots gives a colour coded representation of the focussing ability of the antenna. The direction of maximum focus is down the x-axis and one can see from the scale that the gain (maximum focussing ability) is 8.5 dBi.

The radiation pattern presented in antenna brochures is usually one or more cuts taken from the full radiation pattern. For example, the cuts that would be published for an antenna whose full radiation pattern is shown in Figure 1 would be the x-y plane cut (azimuth) and the x-z plane (elevation).

Figure 2: The azimuth (x-y plane) and elevation (x-z plane) 2D radiation patterns

The y-z plane would generally not be published as the antenna does not radiate much in this plane. The y-z radiation pattern is shown in Figure 3 – note that the maximum radiation in this plane is about -10dBi.



Figure 3: The y-z plane pattern – normally not published

It was mentioned earlier that the gain and radiation pattern should be considered together when assessing an antenna. To illustrate, Figure 4 shows the radiation pattern of antenna whose gain is 8.8 dBi, but has very different radiation characteristics.

Figure 4: An omni-directional radiation pattern

The published patterns of such an antenna would include an elevation cut (any plane that includes the z-axis) and the azimuth (x-y plane)



Figure 5: The azimuth and elevation cuts for the omni-directional antenna

The gain of an antenna is always given with respect to another (well defined) antenna. In other words, the gain of an antenna is always given in units that compare its maximum radiation to the maximum radiation of another antenna (with both antennas having the same input power). The units of gain used here are dBi, where the ‘i’ is used to specify the reference antenna (in this case an isotropic source). So, if the gain is given as 8.5 dBi, then this means that 8.5 dB more power is transmitted in the direction of maximum radiation than that of an isotropic source. Another commonly used unit is dBd, where the‘d’ stands for dipole. If the gain of an antenna is given as 5 dBd, then this means that 5 dB more power is transmitted in the direction of maximum radiation than is transmitted by a dipole in its direction of maximum radiation.

An isotropic source is a source that transmits equally in all directions. Such a device does not exist in reality, but it is very easy to visualise its radiation pattern – it’s just a sphere.

Figure 6: The radiation pattern of an isotropic source



Note that the gain of the isotropic source is given with respect to another isotropic source (i.e., it is compared to itself). That is why the gain of an isotropic source is 0 dBi - so, 0 dB more power is transmitted in the direction of maximum radiation (in this case all directions) than that of an isotropic source.

We cannot directly see the radiation from antennas that work in the frequency bands commonly used for cellular or Wi-Fi communications, so it can be difficult to visualise what the antenna is doing. So let’s consider an antenna where we can see what is going on.

2.1. Consider a lightbulb In this section we are going to introduce an “isotropic” source that we can see and use it to build an antenna. We will plot the radiation pattern of the antenna and compute its gain. This exercise will reinforce the concept that the gain of the antenna is always given with respect to a reference.

It was mentioned earlier that a true isotropic source does not exist in reality, but we can get pretty close. Visualise a lightbulb with a very small battery attached – small enough that it does not impede the radiation of light in any direction. This is our isotropic source which radiates electromagnetic energy at about 100 THz (compared to 2.4 GHz for Wi-Fi).

Let us enclose the lightbulb in an imaginary sphere (say 1m in radius). At each point on the sphere, we can use a light meter to determine the intensity of the light radiated by the bulb. Plotting this data in 3D we should end up with a pattern similar to that in Figure 6.

We can now create a reflector antenna using a shaped piece of metal and lightbulb as shown in Figure 7.

Figure 7: A reflector antenna formed using the isotropic source

The same procedure as before can be used to measure the intensity of the light over the surface of the imaginary sphere. Plotting the results in 3D, one might get something like that shown in Figure 8.



Figure 8: Radiation intensity of the lightbulb with reflector

Let us assume that the highest intensity of light emitted from this structure is 1000 times that of the isotropic source.

If this was the case, then one would be able to make the statement that that 1000 times more power is transmitted in the direction of maximum radiation than that of the isotropic source (the lightbulb on its own).

The gain of an antenna is normally given in decibels. The following formula is used to convert a linear power value to dB:

�� = 10 × ��

30 = 10 × �� 1000�

So, the gain of lightbulb antenna is 30dBi, where the ‘i’ indicates that an isotropic source (the isolated lightbulb) was used as a reference.

The statement can now be made that the gain of the lightbulb antenna is 30 dBi, which implies that 30 dB more power is transmitted in the direction of maximum radiation than that of an isotropic source.

It should be noted that the input power to the bulb was not changed when introducing the reflector. The 30 dBi gain was achieved simply by focussing the light emitted by the lightbulb in a particular direction. The result of focussing the light means that less light is radiated in other directions.

Put another way, if one added up the power of the light radiated in all directions by the bulb antenna and did the same for the isotropic source (lightbulb on its own), then one would come up with the same answer for both cases. This reinforces the fact that the antenna only focussed the energy that was available from the lightbulb. That is all an antenna can do.

Solwise - one of our distributors did a great video explaining gain. https://youtu.be/wGE4tjATecY

Engineering

Antenna parameters part 1: Frequency bands, Gain and Radiation Pattern